1. Field of the Invention
The present invention relates to a power supply device, for example, for an image forming apparatus using an electrophotographic process, and an image forming apparatus using the power supply device.
2. Description of the Related Art
In the technology of an electrophotographic printer, a direct transfer method is known in which a transfer roller is brought into contact with a photoreceptor and an image is transferred onto a record sheet inserted between the photoreceptor and the transfer roller. In this case, roller-shaped conductive rubber having a conductive axis is used as the transfer roller and the roller is driven corresponding to a process speed of the photoreceptor. Then, a direct bias voltage is applied to the transfer roller. Polarities of the direct bias voltage are the same as those of a normal corona discharge transfer voltage. However, to obtain excellent transfer when using such a transfer roller, a voltage equal to or more than 3 kV (prescribed current: several μA) is preferably applied to the transfer roller. To generate such a high voltage, a winding electromagnetic transformer has been conventionally used. However, such a transformer is configured of copper wires, bins, and magnetic cores, and if the transformer is used according to the aforementioned specifications, a leakage current in each section is preferably minimized to the fullest extent possible since an output current value is a micro current of the order of several μA. Thus, the transformer winding is preferably molded with insulating material, and in addition, a large-capacity transformer is needed compared with supply power, which prevents reduction in size and weight of high voltage power supply devices. A typical electrophotographic printer has, in addition to the aforementioned transfer, various units requiring high voltages, for example, for primary charge of a photoconductor drum, development, and as required, fixing bias, and secondary transfer in a case of using an intermediate transfer member. Furthermore, in some color printers, such units are provided for each color station and, as a result, many transformers are mounted. Thus, if winding electromagnetic transformers are used, it is difficult to achieve reduction in size and weight of the whole apparatus.
To overcome these shortcomings, using a thin, light, and high voltage piezoelectric transformer to generate a high voltage has been proposed. As an example, it has been suggested that by using a piezoelectric transformer made of ceramics, a high voltage can be generated more efficiently than an electromagnetic transformer. Moreover, in the case of the piezoelectric transformer, since primary and secondary electrodes can be located away from each other regardless of coupling between them, no special molding for insulation is needed and thus a high voltage power supply can be made smaller and lighter.
FIG. 10 is a diagram illustrating an example of a conventional high voltage power supply circuit using a piezoelectric transformer 101. Such a high voltage power supply circuit is discussed in Japanese Patent Application Laid-Open No. 11-206113.
Reference numeral 101 denotes a piezoelectric transformer (piezoelectric ceramic transformer) of high voltage power supply. Output of the piezoelectric transformer 101 is rectified and smoothed to a positive voltage by diodes 102 and 103 and a high voltage capacitor 104, and then supplied to a transfer roller (not shown), which serves as a load. This output voltage Vout is divided by resistors 105, 106, and 107, and is input as a detection signal (Vsns) to a non-inverting input terminal (positive terminal) of an operational amplifier 109 via a protective resistor 108. On the other hand, a control signal (Vcont) of high voltage power supply, which is an analog signal, is input to an inverting input terminal (negative terminal) of the operational amplifier 109 from a control board via a resistor 114. The operational amplifier 109, resistor 114, and capacitor 113 function as an integrating circuit and a control signal Vcont blunted by an integration time constant that is determined by resistance and capacitance of the resistor 114 and capacitor 113, is input to the operational amplifier 109. In addition, output of the operational amplifier 109 is connected to a voltage controlled oscillator (VCO) 110. Power is supplied to a primary side of the piezoelectric transformer 101 by driving a transistor 111 connected to an inductor 112 using the output of the VCO 110.
FIG. 11 is a diagram illustrating characteristics of the piezoelectric transformer 101.
In the diagram, the characteristics show a trailing shape so that the output voltage is at its maximum at resonant frequency f0 and enables control of the output voltage by frequency. When the output voltage is controlled using frequencies higher than the resonant frequency f0, it is evident from the diagram that drive frequencies should be changed from higher to lower frequencies to raise the output voltage of the piezoelectric transformer 101.
As shown in FIG. 11, in addition to the resonant frequency f0, a spurious resonant frequency (henceforth referred to as a spurious frequency) occurs in the piezoelectric transformer 101 due to structural characteristics of the piezoelectric transformer 101. In a case of sweeping from a sufficiently high drive frequency to a drive frequency of the piezoelectric transformer to obtain a desired output voltage near the resonant frequency f0, a micro output voltage is generated in the course of passing each spurious frequency. This delays the sweep time of frequency and the rise time of a high voltage output becomes longer. More details will be described below.
FIG. 10 shows a configuration in which an output voltage value of the operational amplifier 109 determines an oscillating frequency of the VCO circuit 110. A sweep time is defined as time until the output voltage value of the operational amplifier 109 matches the desired voltage value, that is, until the oscillating frequency of the VCO circuit 110 is fixed to the desired oscillating frequency. The operational amplifier 109 receives the voltage of the control signal Vcont from the control board via the inverting input terminal (negative terminal) and receives the detection signal Vsns via the non-inverting input terminal (positive terminal). Then, an output voltage Vopout of the operational amplifier 109 is determined by a difference of voltage value between the control signal Vcont and detection signal Vsns, and the sweep time of the oscillating frequency is determined by the output voltage Vopout.
FIG. 12 is a diagram illustrating output voltage-time characteristics of a conventional high voltage power supply of the piezoelectric transformer type.
Reference numeral 1201 denotes a waveform of the control signal Vcont of the high voltage power supply and Reference numeral 1202 denotes a waveform of the output voltage Vopout of the operational amplifier 109, and Reference numeral 1203 denotes a waveform of the output voltage Vout. Reference numeral 1210 denotes timing when the high voltage power supply device is turned on, and Reference numerals 1211 and 1212 denote the time when each spurious frequency is passed during the rise time of the output voltage. As shown in Reference numeral 1203, the waveform of the output voltage Vout indicates that a micro output voltage is generated when each spurious frequency (1211 and 1212) is passed in a process of sweeping through drive frequencies. Then, the detection signal value Vsns based on the output voltage value is input to the operational amplifier 109 and the output voltage Vopout (1202) of the operational amplifier 109 rises. Accordingly, the sweep time of the oscillating frequency becomes longer. Thus, the delay of the rise time resulting from spurious frequencies occurs between the time when the high voltage power supply is ON and the control signal Vcont is input to the operational amplifier (1210), and the time when the output voltage reaches the desired output setting voltage.